a. Restoring the default theme
Deleting the Light theme is not a big problem. You can easily recreate it. The Light theme is nothing more than an empty theme set to the Light color-scheme.
So, let’s say your theme list currently looks like this
Here’s what you need to do
Create a new theme by clicking here:
then give it a name and make sure it’s set to theme in the dropdown
I named it “Light” for consistency.
Make sure the color-scheme for your new “Light” theme is set to “Light Scheme” - which it should be by default
Set your new “Light” theme as the default theme for your community
That would be all you need to do to restore the Light theme as the default theme.
b. The problem you were trying to solve when you deleted the Light theme
Are you on the latest version of Discourse?
I know you said
but there was a bug that only affected site admins where the theme set in your user-preferences - as an admin - would continue to be used even if that theme is no longer user-selectable.
That bug was fixed here:
Once you get the Light theme back, can you please verify that you’re still seeing the problem?